1 /* 2 * Copyright (c) 1997, 2021, Oracle and/or its affiliates. All rights reserved. 3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. 4 * 5 * This code is free software; you can redistribute it and/or modify it 6 * under the terms of the GNU General Public License version 2 only, as 7 * published by the Free Software Foundation. 8 * 9 * This code is distributed in the hope that it will be useful, but WITHOUT 10 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or 11 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License 12 * version 2 for more details (a copy is included in the LICENSE file that 13 * accompanied this code). 14 * 15 * You should have received a copy of the GNU General Public License version 16 * 2 along with this work; if not, write to the Free Software Foundation, 17 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. 18 * 19 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA 20 * or visit www.oracle.com if you need additional information or have any 21 * questions. 22 * 23 */ 24 25 #ifndef SHARE_OOPS_MARKWORD_HPP 26 #define SHARE_OOPS_MARKWORD_HPP 27 28 #include "metaprogramming/integralConstant.hpp" 29 #include "metaprogramming/primitiveConversions.hpp" 30 #include "oops/compressedKlass.hpp" 31 #include "runtime/globals.hpp" 32 33 // The markWord describes the header of an object. 34 // 35 // Bit-format of an object header (most significant first, big endian layout below): 36 // 37 // 32 bits: 38 // -------- 39 // hash:25 ------------>| age:4 unused_gap:1 lock:2 (normal object) 40 // 41 // 64 bits: 42 // -------- 43 // nklass:32 hash:25 -->| unused_gap:1 age:4 unused_gap:1 lock:2 (normal object) 44 // 45 // - hash contains the identity hash value: largest value is 46 // 31 bits, see os::random(). Also, 64-bit vm's require 47 // a hash value no bigger than 32 bits because they will not 48 // properly generate a mask larger than that: see library_call.cpp 49 // 50 // - the two lock bits are used to describe three states: locked/unlocked and monitor. 51 // 52 // [ptr | 00] locked ptr points to real header on stack 53 // [header | 01] unlocked regular object header 54 // [ptr | 10] monitor inflated lock (header is wapped out) 55 // [ptr | 11] marked used to mark an object 56 // [0 ............ 0| 00] inflating inflation in progress 57 // 58 // We assume that stack/thread pointers have the lowest two bits cleared. 59 // 60 // - INFLATING() is a distinguished markword value of all zeros that is 61 // used when inflating an existing stack-lock into an ObjectMonitor. 62 // See below for is_being_inflated() and INFLATING(). 63 64 class BasicLock; 65 class ObjectMonitor; 66 class JavaThread; 67 class outputStream; 68 69 class markWord { 70 private: 71 uintptr_t _value; 72 73 public: 74 explicit markWord(uintptr_t value) : _value(value) {} 75 76 markWord() = default; // Doesn't initialize _value. 77 78 // It is critical for performance that this class be trivially 79 // destructable, copyable, and assignable. 80 ~markWord() = default; 81 markWord(const markWord&) = default; 82 markWord& operator=(const markWord&) = default; 83 84 static markWord from_pointer(void* ptr) { 85 return markWord((uintptr_t)ptr); 86 } 87 void* to_pointer() const { 88 return (void*)_value; 89 } 90 91 bool operator==(const markWord& other) const { 92 return _value == other._value; 93 } 94 bool operator!=(const markWord& other) const { 95 return !operator==(other); 96 } 97 98 // Conversion 99 uintptr_t value() const { return _value; } 100 101 // Constants 102 static const int age_bits = 4; 103 static const int lock_bits = 2; 104 static const int self_forwarded_bits = 1; 105 static const int max_hash_bits = BitsPerWord - age_bits - lock_bits - self_forwarded_bits; 106 static const int hash_bits = max_hash_bits > 25 ? 25 : max_hash_bits; 107 #ifdef _LP64 108 static const int klass_bits = 32; 109 #endif 110 111 static const int lock_shift = 0; 112 static const int self_forwarded_shift = lock_shift + lock_bits; 113 static const int age_shift = self_forwarded_shift + self_forwarded_bits; 114 static const int hash_shift = age_shift + age_bits; 115 #ifdef _LP64 116 static const int klass_shift = hash_shift + hash_bits; 117 #endif 118 119 static const uintptr_t lock_mask = right_n_bits(lock_bits); 120 static const uintptr_t lock_mask_in_place = lock_mask << lock_shift; 121 static const uintptr_t self_forwarded_mask = right_n_bits(self_forwarded_bits); 122 static const uintptr_t self_forwarded_mask_in_place = self_forwarded_mask << self_forwarded_shift; 123 static const uintptr_t age_mask = right_n_bits(age_bits); 124 static const uintptr_t age_mask_in_place = age_mask << age_shift; 125 static const uintptr_t hash_mask = right_n_bits(hash_bits); 126 static const uintptr_t hash_mask_in_place = hash_mask << hash_shift; 127 128 #ifdef _LP64 129 static const uintptr_t klass_mask = right_n_bits(klass_bits); 130 static const uintptr_t klass_mask_in_place = klass_mask << klass_shift; 131 #endif 132 133 static const uintptr_t locked_value = 0; 134 static const uintptr_t unlocked_value = 1; 135 static const uintptr_t monitor_value = 2; 136 static const uintptr_t marked_value = 3; 137 138 static const uintptr_t no_hash = 0 ; // no hash value assigned 139 static const uintptr_t no_hash_in_place = (address_word)no_hash << hash_shift; 140 static const uintptr_t no_lock_in_place = unlocked_value; 141 142 static const uint max_age = age_mask; 143 144 // Creates a markWord with all bits set to zero. 145 static markWord zero() { return markWord(uintptr_t(0)); } 146 147 // lock accessors (note that these assume lock_shift == 0) 148 bool is_locked() const { 149 return (mask_bits(value(), lock_mask_in_place) != unlocked_value); 150 } 151 bool is_unlocked() const { 152 return (mask_bits(value(), lock_mask_in_place) == unlocked_value); 153 } 154 bool is_marked() const { 155 return (mask_bits(value(), lock_mask_in_place) == marked_value); 156 } 157 bool is_neutral() const { 158 return (mask_bits(value(), lock_mask_in_place) == unlocked_value); 159 } 160 161 // Special temporary state of the markWord while being inflated. 162 // Code that looks at mark outside a lock need to take this into account. 163 bool is_being_inflated() const { return (value() == 0); } 164 165 // Distinguished markword value - used when inflating over 166 // an existing stack-lock. 0 indicates the markword is "BUSY". 167 // Lockword mutators that use a LD...CAS idiom should always 168 // check for and avoid overwriting a 0 value installed by some 169 // other thread. (They should spin or block instead. The 0 value 170 // is transient and *should* be short-lived). 171 static markWord INFLATING() { return zero(); } // inflate-in-progress 172 173 // Should this header be preserved during GC? 174 bool must_be_preserved(const oopDesc* obj) const { 175 return (!is_unlocked() || !has_no_hash()); 176 } 177 178 // WARNING: The following routines are used EXCLUSIVELY by 179 // synchronization functions. They are not really gc safe. 180 // They must get updated if markWord layout get changed. 181 markWord set_unlocked() const { 182 return markWord(value() | unlocked_value); 183 } 184 bool has_locker() const { 185 return ((value() & lock_mask_in_place) == locked_value); 186 } 187 BasicLock* locker() const { 188 assert(has_locker(), "check"); 189 return (BasicLock*) value(); 190 } 191 bool has_monitor() const { 192 return ((value() & lock_mask_in_place) == monitor_value); 193 } 194 ObjectMonitor* monitor() const { 195 assert(has_monitor(), "check"); 196 // Use xor instead of &~ to provide one extra tag-bit check. 197 return (ObjectMonitor*) (value() ^ monitor_value); 198 } 199 bool has_displaced_mark_helper() const { 200 return ((value() & unlocked_value) == 0); 201 } 202 markWord displaced_mark_helper() const; 203 void set_displaced_mark_helper(markWord m) const; 204 markWord copy_set_hash(intptr_t hash) const { 205 uintptr_t tmp = value() & (~hash_mask_in_place); 206 tmp |= ((hash & hash_mask) << hash_shift); 207 return markWord(tmp); 208 } 209 // it is only used to be stored into BasicLock as the 210 // indicator that the lock is using heavyweight monitor 211 static markWord unused_mark() { 212 return markWord(marked_value); 213 } 214 // the following two functions create the markWord to be 215 // stored into object header, it encodes monitor info 216 static markWord encode(BasicLock* lock) { 217 return from_pointer(lock); 218 } 219 static markWord encode(ObjectMonitor* monitor) { 220 uintptr_t tmp = (uintptr_t) monitor; 221 return markWord(tmp | monitor_value); 222 } 223 224 // used to encode pointers during GC 225 markWord clear_lock_bits() { return markWord(value() & ~lock_mask_in_place); } 226 227 // age operations 228 markWord set_marked() { return markWord((value() & ~lock_mask_in_place) | marked_value); } 229 markWord set_unmarked() { return markWord((value() & ~lock_mask_in_place) | unlocked_value); } 230 231 uint age() const { return mask_bits(value() >> age_shift, age_mask); } 232 markWord set_age(uint v) const { 233 assert((v & ~age_mask) == 0, "shouldn't overflow age field"); 234 return markWord((value() & ~age_mask_in_place) | ((v & age_mask) << age_shift)); 235 } 236 markWord incr_age() const { return age() == max_age ? markWord(_value) : set_age(age() + 1); } 237 238 // hash operations 239 intptr_t hash() const { 240 return mask_bits(value() >> hash_shift, hash_mask); 241 } 242 243 bool has_no_hash() const { 244 return hash() == no_hash; 245 } 246 247 #ifdef _LP64 248 inline Klass* klass() const; 249 inline Klass* klass_or_null() const; 250 inline Klass* safe_klass() const; 251 inline markWord set_klass(const Klass* klass) const; 252 inline narrowKlass narrow_klass() const; 253 inline markWord set_narrow_klass(const narrowKlass klass) const; 254 #endif 255 256 // Prototype mark for initialization 257 static markWord prototype() { 258 return markWord( no_hash_in_place | no_lock_in_place ); 259 } 260 261 // Debugging 262 void print_on(outputStream* st, bool print_monitor_info = true) const; 263 264 // Prepare address of oop for placement into mark 265 inline static markWord encode_pointer_as_mark(void* p) { return from_pointer(p).set_marked(); } 266 267 // Recover address of oop from encoded form used in mark 268 inline void* decode_pointer() { return (void*)clear_lock_bits().value(); } 269 270 inline bool self_forwarded() const { 271 return mask_bits(value(), self_forwarded_mask_in_place) != 0; 272 } 273 274 inline markWord set_self_forwarded() const { 275 return markWord(value() | self_forwarded_mask_in_place | marked_value); 276 } 277 }; 278 279 // Support atomic operations. 280 template<> 281 struct PrimitiveConversions::Translate<markWord> : public TrueType { 282 typedef markWord Value; 283 typedef uintptr_t Decayed; 284 285 static Decayed decay(const Value& x) { return x.value(); } 286 static Value recover(Decayed x) { return Value(x); } 287 }; 288 289 #endif // SHARE_OOPS_MARKWORD_HPP